Aromatics, particularly benzene, toluene, ethylbenzene, and the xylenes (ortho, meta, and para isomers), which are commonly referred to as “BTEX” or more simply “BTX,” are extremely useful chemicals in the petrochemical industry. They represent the building blocks for materials such as polystyrene, styrene-butadiene rubber, polyethylene terephthalate, polyester, phthalic anhydride, solvents, polyurethane, benzoic acid, and numerous other components. Conventionally, BTX is obtained for the petrochemical industry by separation and processing of fossil-fuel petroleum fractions, for example, in catalytic reforming or cracking refinery process units, followed by BTX recovery units.
Typically, integrated refining-petrochemical complexes separate a crude feedstock into a “straight run” or desired fraction of naphtha, such as C6-C10 naphtha, i.e., naphtha containing hydrocarbons having six to ten carbon atoms, and a heavier fraction containing longer chain hydrocarbons such as heavy oils and residues. The naphtha stream typically undergoes reforming to produce a reformate stream with an increased aromatic content. The reformate stream is processed in an aromatics complex to produce selected aromatic products, such as benzene and para-xylene.
Some of the naphtha, typically about 5-20 wt % of the C5+ reformer feed stream remains as aliphatic C5+ hydrocarbon in the reformate after the reforming reaction. The reformate is separated from the C4− components in a flash separator and a debutanizer column, and then split in a reformate splitter column between the C8− fraction having the majority of the aliphatics and the C8+ fraction having a higher concentration of aromatics than the C8− fraction. The aliphatics are in the C8− fraction which are separated in an extractive unit, which commonly uses Sulfolane solvent, as a raffinate from the aromatic benzene and toluene. The benzene is further fractionated as a pure petrochemical product. The raffinate is then normally blended into gasoline or sent to a thermal cracking unit to produce ethylene, propylene, and heavier components.
The reformate splitter column and the extractive unit, which may include three columns, are expensive to build and to operate. In addition, the utilization of the raffinate stream from the extractive distillation unit rich in aliphatics or light boiling naphtha streams with T95 boiling point of less than 82° C. is often difficult for gasoline blending as it has relatively low octane number and high vapor pressure or for thermal steam cracking to convert to ethylene and propylene because it requires a large investment in a downstream cracking unit.
Therefore, there is a need for processes for converting naphtha into aromatics which reduce capacity of expensive distillation columns and improve the value of the raffinate stream and other light naphthas at lower capital cost.